1. Field of the Invention
The present invention relates to an image forming apparatus that is configured to measure toner bearing amounts of at least two toner images, which were formed for measurement purposes and which have different toner image forming conditions, to set a toner image forming condition for image formation, and more particularly, to a control for suppressing density change in a halftone image before and after the toner image forming condition is set.
2. Description of the Related Art
An image forming apparatus is widely used, which is configured to develop an electrostatic image formed on an image bearing member into a toner image by a developing device. The apparatus transfers the developed toner image directly, or via an intermediate transfer member, onto a recording material. The apparatus also uses a fixing device to fix the image onto the recording material by heating and pressurizing the recording material, on which the toner image has been transferred.
In the image forming apparatus, in order to enhance the reproducibility of the image density of the fixed image, a setting mode is executed prior to image formation, in order to set a setting condition for electrically defining the developability of the toner image between a developer carrying member and the image bearing member at the time of image formation.
In Japanese Patent Application Laid-Open No. 2001-42580, in the setting mode, a predetermined developing contrast is set in the image forming apparatus to form a toner image for measurement (patch image), and a toner bearing amount of the toner image for measurement is measured with the use of an optical sensor disposed opposite to the image bearing member or the intermediate transfer member. The developing contrast refers to a potential difference (see FIG. 3) between the potential of an image section, of an electrostatic image formed on the image bearing member, and a DC voltage to be applied to the developer carrying member.
Then, in order to obtain a toner image having a certain target toner bearing amount based on the result of measurement of the toner bearing amount of the toner image formed for measurement, the developing contrast for the image formation, and the electrostatic image forming condition or the developing condition to obtain the developing contrast are adjusted.
In Japanese Patent Application Laid-Open No. 2001-42580, the setting mode is executed in a state in which a toner image formed for measurement and having a toner bearing amount close to the maximum density that can be output as the fixed image is formed on the image bearing member. This is because the maximum density of the fixed image to be output in the image formation is ensured.
However, when the setting mode is executed using the toner image formed for measurement and having the toner bearing amount close to the maximum density, the density of an image having maximum density is kept equal before and after the setting mode is executed, but the image density of a halftone image varies significantly in some cases. For example, in a case where the developability of the developing device decreases, due to the deterioration of toner, change of temperature and humidity, or the like, when the setting mode is executed to equally set the maximum density of an output image, the reproducibility of the image density of the halftone image is significantly diminished in some cases (Comparative Example 1).
In this case, it is possible to measure the toner bearing amounts of the toner images formed for measurement having a plurality of levels of medium gray scale, and to perform gray scale conversion, called gamma correction, to cancel the difference in image density generated in the medium gray scale (Comparative Example 2). However, when gamma correction is executed, a down-time therefor is generated in the image forming apparatus.